1. Field of the Invention
The present invention relates to prostheses adapted to be fixedly attached to bone by means of bone cement. Specifically, the present invention is directed to prostheses adapted to maximize the durability of the prosthesis-bone cement adherence.
2. Description of the Prior Art
In the field of orthopedic surgery, metal alloys such as ZIMALOY manufactured by Zimmer U.S.A., Inc., a chromium-cobalt-molybdenum alloy, stainless steel and titanium alloys, and highly rigid plastics such as ultra-high molecular weight polyethylene (hereinafter UHMWPE) have been used successfully to replace the ends of long bones and joints including the hip joint. However, there exists a severe limitation with respect to this type of orthopedic surgery, namely, coupling of the prostheses to bone. Due to such factors as mechanical stress, fatigue, corrosion, etc., the prostheses-bone cement joints have been prone to failure. As disclosed in applicant's co-pending patent application, Ser. No. 45,657, filed June 5, 1979; entitled "Bone Connective Prostheses Adapted to Maximize Strength and Durability of Prosthese-Bone Cement Interface; and Methods of Forming Same", improved bone-connective prostheses may be prepared by treating a rigid prosthetic element, adapted to be joined to bone by means of bone cement, to provide an adherent polymethylmethacrylate film on the surface thereof. Prosthetic elements bearing a polymethylmethacrylate film as disclosed in applicant's co-pending application can more readily be joined to bone cement and provide a stronger and more durable joint.
However, an additional problem with respect to bone connective prostheses relates to the differences between the rigidity of the prosthetic element and the bone cement. Thus, prosthetic elements designed to be attached to bone, e.g., stem insert prostheses and bone resurfacing prostheses, are generally composed of a very rigid material such as a metal alloy or UHMWPE. On the other hand, bone cement, which normally comprises a mixture of polymethylmethacrylate and methyl methacrylate monomer and which may additionally include a styrene copolymer of methyl methacrylate, is typically a less rigid, porous material. Accordingly, when such a rigid prosthetic element is joined to bone by means of bone cement and the resultant composite subjected to force, e.g., walking in the case of a hip ball joint implant, the interface between the rigid implant and the less rigid bone cement is subjected to a high degree of mechanical stress. Moreover, extreme non-physiological force distribution (in the bone) can occur as well as extreme stress concentration in the prosthesis-cement complex, resulting in bone atrophy, weakening of the bone, loss of interfacial integrity and subsequent fracture of one or more of the components. Additionally, these forces can result in a fatigue or brittle fracture of the bone cement since the cement typically includes flaws such as voids, thus leading to failure of the prosthesis.